Suggest: keep completionModel alive in the suggestModel to speed things up dramatically in the no matching suggestions case

fixes #1260

Add gulp tasks to build deb packages

fixes #2291

Build, install and run:
gulp vscode-linux-packages
sudo dpkg -i out-linux/vscode-amd64.deb
code .

Installing the package does the following:
- Puts VSCode dir at /usr/share/code
- Puts code.sh launcher in /usr/bin
- Defines a .desktop file to properly integrate with the launcher

Fixes #2679

fixes #2302

fixes #2086

fixes #2280